1. Field of the Invention
The present invention relates to a plurality of pump-turbines wherein an upper reservoir-side conduit or a lower reservoir-side conduit is branched and the conduit portions located farther than the branch points are shared among the pump-turbines. Particularly, the invention is concerned with a plurality of pump-turbines capable of diminishing water hammer.
2. Description of the Related Art
Generally, in conventional pump-turbines, different guide vane closing speeds are set for pumping operation and generating operation, respectively. For the closing speed at each operation mode there is adopted a closing method involving a so-called gooseneck in which the closing speed is changed over to a slow closing speed automatically when guide vanes have been closed to a predetermined opening or less. The reason for this will be described below while referring to a generating operation mode shown in FIG. 7.
During a generating operation at a guide vane opening corresponding to a full load or a load close thereto, if the load of a generator connected directly to a pump-turbine is disconnected suddenly, that is, if the load is rejected, the rotational speed of the pump-turbine will rise temporarily. In an ordinary type of a pump-turbine, however, it is necessary to also fully satisfy the required characteristics in pumping operation in which the rotation is reverse, so that the diameter of each runner vane is set relatively large and some consideration is given to the runner shape so that a sufficient centrifugal force acts on water. Therefore, also in the generating operation, the influence of the centrifugal force acting on the water flowing down through the runner is serious. With an increase of the rotational speed, the amount of water entering the runner chamber decreases at a gradient which is far steeper than that in an ordinary type of a turbine, eventually leading to a reverse flow, namely the pumping flow. Thus, even without closing of the guide vanes, a mere increase in rotational speed of the turbine results in a steep gradient decrease of the turbine discharge. The higher the head of pump, the more marked this tendency. Particularly, in the case of a pump-turbine having an S-characteristic (dQ1/dN&gt;0, where Q1=Q/.sqroot. H, N1=N/.sqroot. H, Q: turbine discharge, N: rotational speed, H: effective head), the rotational speed increases, reaches a peak, then decreases, and just after this decrease there occurs a phenomenon such that the flow in the turbine direction suddenly shifts to the flow in the pump direction as if it were pulled in. This is due to a positive feedback phenomenon such that the increase of an effective head H caused by water hammer decreases the flow rate Q, giving rise to a further water hammer and causing a further increase of the effective head H, as is fully explained in Japanese Patent Laid Open No. 40946/79. As a result, the water pressure in the upper reservoir-side conduit of the pump-turbine increases abruptly. In contrast therewith, the water pressure in the lower reservoir-side conduit of the pump-turbine drops largely. If the rapid closing of guide vanes continues even at this time, this state indicates an overlapped form of the pumping action, especially a natural flow decreasing action based on the S-characteristic, with a rapid throttling effect induced by the guide vanes. This state is dangerous because the increase of water pressure in an iron penstock expands to an abnormal extent. The water pressure in the upper reservoir-side conduit rises to an abnormal extent and may exceed a designed level, while the water pressure on the lower reservoir-side drops to an abnormal extent and may cause a water column separation. The reason for this is explained in Japanese Patent Laid Open No. 143841/78, but is apparent from a complete characteristic graph of a pump-turbine shown in FIG. 8. As shown therein, closing the guide vanes while following the S-characteristic in a flow decreasing direction means that an operation point N1=N/.sqroot. R shifts in a direction in which it becomes smaller. This is because a rise of H is inevitable on the assumption that N is constant. The broken line in FIG. 7 shows an example, in which an increasing range of water pressure Hp in the upper reservoir-side conduit increases although an increasing range of rotational speed becomes a little smaller.
According to the prior art, therefore, at a guide vane opening smaller than a predetermined opening, say 80%, in the generating operation mode, the upper limit of the guide vane closing speed is set smaller than the upper limit of the closing speed at a guide vane opening of 80% or more, allowing the closing speed to shift to a slower speed and thereby allowing the guide vane closing pattern to be goosenecked as shown by line 40 in FIG. 7. For example, if there occurs a load rejection (time t0) at a guide vane opening close to 100%, the guide vanes close in a relatively rapid manner at the beginning, and at a time point ta corresponding to the arrival of the guide vanes at a preset opening Ya, the closing speed limit is changed over to a smaller value. Therefore, during the entry of an operation point into the S-characteristic and descent in the flow decreasing direction which operation point begins at the time when the rotational speed of the pump-turbine exceeds its maximum value and turns to decrease, the guide vane closing speed is limited to a relatively slow speed, whereby the foregoing abnormal increase of water pressure can be prevented. The solid line in FIG. 7 represents the state of the water pressure Hp in the iron penstock and that of the rotational speed N in the case where the guide vane closing speed is goosenecked. As means for changing the guide vane closing speed according to the guide vane opening there is used a guide vane closing speed selector. For example, such a pump-turbine protecting device as disclosed in Japanese Patent Publication No. 38559/85 is known.
In the device disclosed in the Japanese patent publication, no consideration is given to a countermeasure to be taken in the event of failure of the guide vane closing speed selector itself or of a guide vane opening detecting means. In view of this point there also has been proposed a pump-turbine in which even in the event of such failure a guide vane gooseneck equal to that responsive to the guide vane opening mentioned above is ensured to ensure the safety of the pump-turbine in the state of load rejection. This known example is disclosed in Japanese Patent Laid Open No. 42441/96, in which it is intended to ensure a guide vane gooseneck even in the event the gooseneck responsive to the guide vane opening fails to operate, thereby ensuring the pump-turbine concerned, insofar as the rotational speed of the pump-turbine is above a predetermined value in the state of load rejection.
It is known that the relation between a guide vane closing pattern in the state of load rejection and water hammer, especially the rise of the water pressure Hp in the upper reservoir-side conduit is such as that shown in FIG. 6. More specifically, if the guide vane opening Ya, which serves as a condition in changing over the guide vane closing speed from a rapid closing to a slow closing, is increased, the first peak value Hpx of the water pressure Hp in the upper reservoir-side conduit drops into Hpx1, while the second peak value Hpy rises into Hpy1. Although the waveform of the water pressure Hd in the lower reservoir-side conduit is not shown, it is such a waveform as is obtained by turning the Hp waveform upside down. The second peak value Hdy1 is lower than Hdy. The Hp waveform changes also upon change of the speed limit on the rapid guide vane closing portion. That is, if the limit is made to a gentler gradient, the first peak value Hpx drops and the second peak value Hpy rises. The most typical example is the case where the rapid closing speed is equal to a slow closing speed lower than the gooseneck point. As explained in Japanese Patent Laid Open No. 40946/79 and as will be seen from the foregoing, the first peak value can be adjusted by adjusting the gooseneck opening and limiting the rapid closing speed, but as to the second peak value, it is very difficult to control it because it results from the operation point being pulled naturally into the S-characteristic. That is, the water pressure Hpy in the upper reservoir-side conduit is allowed to take its own course and it is very difficult to control it. Particularly, in the case where a plurality of pump-turbines having the S-characteristic share conduits as in FIG. 2, they undergo a water-hammer interference with each other through the common portions of the conduits, so it is difficult to estimate the value of the water pressure Hpy in the upper reservoir-side conduit in the case where the loads on the plural pump-turbines are rejected successively with a time difference.
It is by no means possible to test all the cases of innumerable combinations in the field, so as to the time difference load rejection, it relies on simulation analysis. However, the S-characteristic itself measured by a pump-turbine model test involves a problem in point of accuracy and so it is impossible to expect a high accuracy. After all, a design is made so as to avoid such a setting as induces an increase of the second peak value of water pressure on the upper reservoir-side like the dotted line in FIG. 6 resulting from a load rejection in a single pump-turbine and so as to make the first peak value sufficiently high like the solid line, at least bring it to the same level as the second peak value. In this case, a consideration is given so that no matter how high the Hpy in time difference load rejection rises abnormally due to mutual interference, it does not exceed Hpx .SIGMA. in the state of simultaneous load rejection of plural pump-turbines which share a conduit and so that a possible maximum water pressure is controlled at the first peak value which can be controlled, not allowing it to take its own course in accordance with the S-characteristic. Usually, if only the relation of Hpx&gt;Hpy is satisfied in the state of load rejection of a single pump-turbine, the relation of Hpx .SIGMA.&gt;Hpy .SIGMA. is retained also at the time of simultaneous rejection of full load (of course, Hpx .SIGMA. in simultaneous full load rejection&gt;Hpx in single machine load rejection). The above prior art still involves the problem that even if the guide vane closing pattern is set so as to meet the relation of Hpx&gt;Hpy like the solid line in FIG. 6 in the state of single machine load rejection, if both are close to each other, there is no guarantee of preventing the increase of the second peak and reversal at the time of load rejection with a time difference. Rather, in the case where the guide vane closing pattern is set so as to meet the relation of Hpx&lt;Hpy in the state of a single machine load rejection, the maximum value Hpy of the water pressure in the upper reservoir-side conduit relies completely on the S-characteristic, so that the possibility of the water pressure in the upper reservoir-side conduit at the time of load rejection with a time difference exceeding the maximum value Hpy .SIGMA. in the state of full load rejection becomes higher.
As the method for detecting that the point of operation is following the S-characteristic in the flow decreasing direction, there is known such a method as disclosed in Japanese Patent Publication No. 21033/88. For example, the detection is made on the basis of an AND condition of dN/dt&lt;0 and d2N/dt2&lt;0 and an AND condition of dN/dt&lt;0 and N&gt;Na.
In Japanese Patent Laid Open No. 134949/77 there is shown an example of a water hammer interference diminishing method for plural pump-turbines sharing conduits. According to this known method, while the load of a certain pump-turbine is rejected and the guide vanes thereof are closing rapidly, a limitation is made on the guide vanes of the other pump-turbines which share conduits so as to close slowly. However, this known method lacks the understanding that the abnormal water-hammer interference is based on the S-characteristic. The operation monitoring item in the pump-turbines other than the pump-turbine concerned is limited to only the guide vane closing speed. As a matter of course, the monitor items proposed in the present invention such as monitoring approach of other machines to the S-characteristic or the state of following the S-characteristic in the flow decreasing direction are not disclosed in the aforesaid unexamined publication. According to the known method in question, the simultaneous rapid closing of the guide vanes in plural pump-turbines sharing conduits cannot be done even in the absence of any fear of danger.
At present there is no appropriate measure against an abnormal water-hammer interference based on the S-characteristic occurring among plural pump-turbines which share conduits, particularly an abnormal rise of Hpy (abnormal drop of Hdy) in the state of load rejection with a time difference. Therefore, this point is to be improved. Besides, although an appropriate consideration has not been given to the S-characteristic and hence it has so far been impossible to minimize water hammer, the present invention permits the minimization of water hammer, namely the improvement of design water pressure in the upper and lower reservoir-side conduits and pump-turbine devices.